- threading: main loop spawns run_one in a thread per queued job; up to len(WORKERS) concurrent.
- pick_worker: thread-safe VRAM reservation to avoid two threads picking the same GPU.
- pop_queued/pop_queued_stitch: atomic SELECT+UPDATE sous BEGIN IMMEDIATE (status claimed).
- Heartbeat daemon: thread qui ecrit dispatcher.heartbeat toutes les 5s (fini le faux dead pendant les jobs longs).
- run_one: libere la reservation VRAM sur finally (error/done/queued).
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